This invention relates to a thickness-shear crystal vibrator having improved vibration characteristics.
An At-cut quartz vibrator is a typical vibrator which has been conventionally known as the thickness-shear crystal vibrator. The AT-cut quartz vibrator has various forms, such as rectangular, circular and bar. Generally, as shown in FIG. 1 (a) and (b), Y axis designates an axis perpendicular to an X axis (an electrical axis) and Z axis (a light axis) of a quartz crystal, wherein it comprises a pair of cut planes (Y' planes) which partially meet at right angles with the Y axis rotated a fixed angles .theta. (about 35.degree. ) around the X axis as a center (Y' axis). The AT-cut quartz vibrator 1 produces a thickness-shear strain and generates a principal vibration of a thickness-shear vibration within a prescribed frequency when an electric field is applied through driving electrodes 2 formed at a pair of the Y' plane above mentioned. Moreover, a contour vibration by way of a spurious vibration which vibrates with a frequency other than the said frequency is also generated according to the form and the dimensions of the vibrator. The distribution of the principal vibration of the AT-cut quartz vibrator effected by the thickness-shear strain is ideally maximum at the center portion of Y' planes measured along the X axis as shown in FIG. 1 (c) and (d), and gradually decreases toward the periphery of the Y' planes, and finally as arrows in FIG. 1 (a) indicate, theoretically becoming zero at a thickness center line (a center axis line or nodal line) 3, which meets at right angles with the Y' axis and which is defined by lines of intersection of a nodal plane of vibration intersecting side surfaces of the vibrator. While as FIG. 1 (c) shows, the distribution of the thickness-shear strain is uniform in a direction of the Z' axis of the At-cut quartz vibrator 1.
Conventionally it has been proposed to miniaturize an AT-cut quartz vibrator by diminishing the width in a direction of the Z' axis. FIG. 2 (a) shows an embodiment, wherein a driving electrode 5 is disposed in the Y' plane of an AT-cut quartz vibrator 4. On the occasion of extending the lead wire from the driving electrode 5 to connect an external electric circuit, the vibration of the AT-cut quartz vibrator 4 should not be disturbed as fas as possible. As shown in FIG. 2(a) an electrode lead 6' extends along the vibrator surface from the side of the driving electrode 5, and a lead portion 6ato connect to the external circuit is connected to the circuit by means of wire bonding method or supporting members which function as a lead wire. However, since the electrode lead 6' is formed along the surface which vibrates with greater amplitude than the center line and adjacent areas, the friction with the vibrator is so great as to be influential in ageing. Therefore, on the occasion the end 6a'is supported, it is disadvantageous because the dissipation caused by supporting, characteristic dispersion and influences on the frequency ageing is increased. These disadvantages becomee more serious practically when the miniaturized vibrator is used for a portable watch or the like, since the relative dimension of supporting members and lead wire in relation to the vibrator are enlarged.
As an improvement, a method as shown in FIG. 2 (b) is proposed, wherein an electrode lead 6 extends from the said driving electrode 5 to the side surface or plane adjacent to the Y' plane along a thickness center line (nodal line or thickness center axis line) 7 defined by the intersection of the vibration nodal plane with the side plane. By forming the electrode lead 6 at the thickness center line 7 of the side plane as shown in FIG. 2 (b), most of the electrode lead is located at the portion where a vibration caused by the strain of the AT-cut quartz vibrator 4 is hard to be conveyed to the lead, wherefore an obstruction to the principal vibration effected by the said electrode lead 6 is not serious, and as a result, a decline of temperature-characteristics, Q factor (a sharpness of resonance) and increase in frequency ageing can be prevented.
However in the AT-cut quartz vibrator 4, the Y' plane, whereat the driving electrode 5 is formed, and the XY' plane, and its adjacent side plane (a plane partially meeting at right angles with the Z' axis and which comprises the X axis and Y' axis, which hereinafter indicated as the Z' plane) meet approximately at right angles, wherefore it is difficult to form the said electrode lead at the edge portion (the adjacent portion) of the Y' plane and Z' plane on the occasion that the electrode lead 6 extends from the said Y' plane to the Z' plane by means of the former metallic film formed by measures such as evaporation and sputtering. As a result, the metallic film to be the electrode lead 6 is hard to adhere to the edge portion of the Y' plane and Z' plane strongly enough to get a high reliability, wherefore unfavorable phenomena result such as increase in electric resistance of the electrode lead 6, and poor connection mechanical results, and efficiency percentage is not good. Therefore, in order to obtain a high reliability, it is required to enlarge the width and the thickness of the electrode lead 6 to help the metalic film to be fully adhered to the edge portion of the said Y' plane and the Z' plane. However, on the occasion that the width and the thickness of the electrode lead 6 is enlarged too much, the friction dissipation of the AT-cut quartz vibrator 4 and the electrode lead 6 increases and internal strain within the electrode lead 6 increases, wherefore an increase in crystal impedance ( CI ), a fall in driving efficiency and an increase in frequency ageing are brought about so as to degenerate the vibration-characteristics.